Wheel supporting rolling bearing unit

ABSTRACT

In a wheel supporting rolling bearing unit, an inner end opening out of both end openings of a space in which balls  14, 14  are provided is sealed with a cap  17   a . An outer end opening is sealed with a seal ring  16   c  having three seal lips. A rolling resistance that changes based on a preload is regulated in a range of 0.15 to 0.45 N·m, and a running resistance of the seal ring  16   c  based on a friction between the seal ring and a counter surface is regulated in a range of 0.03 to 0.2 N·m.

TECHNICAL FIELD

The present invention relates to improvement in a wheel supportingrolling bearing unit for supporting rotatably a wheel, especially anidler wheel (a rear wheel of an FF car, a front wheel of an FR car andan RR car), on a suspension system of a vehicle (car).

BACKGROUND

As the wheel supporting rolling bearing unit, structures shown in FIGS.11 and 12 are set forth in JP-A-2001-221243, for example. First, astructure of a first example shown in FIG.11 will be explainedhereunder. A wheel 1 constituting the wheel is supported rotatably on anend portion of an axle 3 constituting a suspension system by a wheelsupporting rolling bearing unit 2. More particularly, inner rings 5, 5as a stationary side raceway ring, which constitutes the wheelsupporting rolling bearing unit 2, are fitted onto a supporting shaft 4fixed to the end portion of the axle 3, and then the inner rings 5, 5are fixed with a nut 6. Meanwhile, the wheel 1 is coupled/fixed to a hub7 as a rotary side raceway ring, which constitutes the wheel supportingrolling bearing unit 2, by a plurality of stud bolts 8, 8 and nuts 9, 9.

Double row outer ring raceways 10 a, 10 b that act as a rotary sideraceway surface respectively are formed on an inner peripheral surfaceof the hub 7, and a fitting flange 11 is formed on an outer peripheralsurface of the same. The wheel 1 as well as a drum 12 constituting abaking system is coupled/fixed to a one-side surface (an outside surfacein the first example, a left side surface in FIGS. 11 and 12) of thefitting flange 11 by the stud bolts 8, 8 and nuts 9, 9.

Balls 14, 14 are provided rollably between the outer ring raceways 10 a, 10 b and inner ring raceways 13, 13, which are formed on outerperipheral surfaces of the inner rings 5, 5 to act as the stationaryside raceway surface respectively, every plural pieces in a state thatthese balls 14, 14 are held in cages 15, 15 respectively. A double rowangular contact ball bearing having a back-to-back arrangement isconstructed by combining respective constituent members mutually in thismanner, so that the hub 7 is supported rotatably around the inner rings5, 5 to bear the radial load and the thrust load. In this case, sealrings 16 a, 16 b are provided between inner peripheral surfaces of bothend portions of the hub 7 and outer peripheral surfaces of end portionsof the inner rings 5, 5 to isolate a space in which the balls 14, 14 areprovided from the external space. In addition, an opening portion of anouter end (Here, an “outside in the axial direction” means the outsideof the hub in the width direction when such hub is fitted to thevehicle. Similarly, an “inside” means the inside of the hub on themiddle side in the width direction.) of the hub 7 is covered with a cap17. The cap 17 has no portion that slidingly comes into contact with theinner rings 5, 5 as the other raceway ring and the axle 3 and the nut 6that are stationary portions with respect to the inner rings 5, 5.

Upon using the above wheel supporting rolling bearing unit 2, as shownin FIG. 11, the supporting shaft 4 onto which the inner rings 5, 5 arefitted and fixed is fixed to the axle 3 and also the wheel 1, to which atire (not shown) is fitted, and the drum 12 are fixed to the fittingflange 11 of the hub 7. Also, a braking drum brake is constructed byassembling in combination the drum 12, a wheel cylinder and shoes (notshown) supported on a backing plate 18 fixed to the end portion of theaxle 3. Upon braking, a pair of shoes provided to the inner diameterside of the drum 12 are pushed against an inner peripheral surface ofthe drum 12.

Next, a structure of a second example shown in FIG.12 in the prior artwill be explained hereunder. In the case of this wheel supportingrolling bearing unit 2 a, a hub 7 a as the rotary side raceway ring issupported rotatably by a plurality of balls 14, 14 on the inner diameterside of an outer ring 19 as the stationary side raceway ring. Therefore,the double row outer ring raceways 10 a , 10 b as the stationary sideraceway surface are provided on the inner peripheral surface of theouter ring 19 respectively, and first and second inner ring raceways 20,21 as the rotary side raceway surface are provided on the outerperipheral surface of the hub 7 a respectively.

The hub 7 a is constructed by using a hub main body 22 as a main shaftmember and an inner ring 23 in combination. A fitting flange 11 a forsupporting the wheel is provided to an outer end portion of the outerperipheral surface of the hub main body 22, and the first inner ringraceway 20 is provided to an middle portion thereof, and also asmall-diameter stepped portion 24 that is smaller in diameter than aportion in which the first inner ring raceway 20 is formed is providedto the middle portion thereof near an inner end. Then, the inner ring23, to an outer peripheral surface of which the second inner ringraceway 21 having a circular-arc sectional shape is provided, is fittedonto the small-diameter stepped portion 24. In addition, an inner endsurface of the inner ring 23 is pressed with a caulking portion 25 thatis formed by elastically deforming an inner end surface of the hub mainbody 22 outward in the radial direction. Thus, the inner ring 23 isfixed to the hub main body 22. Further, seal rings 16 c, 16 d areprovided between an inner peripheral surface of the outer ring 19 onboth end portions and the outer peripheral surface of the middle portionof the hub 7 a and the outer peripheral surface of the inner end portionof the inner ring 23 respectively. Thus, the spaces in which the balls14, 14 are provided are isolated from the external space between theinner peripheral surface of the outer ring 19 and the outer peripheralsurface of the hub 7 a.

In this case, in the case of the above wheel supporting rolling bearingunit 2 a shown in FIG. 12, the rigidity can be enhanced because thefirst inner ring raceway 20 is formed directly on the outer peripheralsurface of the middle portion of the hub main body 22. In other words,the first inner ring raceway to be provided in the middle portion of thewheel supporting rolling bearing unit can be formed on the outerperipheral surface of the inner ring prepared as a separate body of thehub main body, and then this inner ring can be fitted/fixed onto the hubmain body. In this case, unless an inference of the inner ring into thehub main body is increased, the rigidity is lowered, like a structureshown in FIG. 12, rather than the case where the first inner ringraceway 20 is formed directly on the outer peripheral surface of themiddle portion of the hub main body 22. A working of fitting the innerring as the separate body onto the hub main body from the inner endportion to the middle portion while keeping a large inference istroublesome. In contrast, as shown in FIG. 12, the wheel supportingrolling bearing unit 2 a having the high rigidity can be constructedwithout trouble by employing the structure in which the first inner ringraceway 20 is formed directly on the outer peripheral surface of themiddle portion of the hub main body 22.

In the case of the above conventional structure, it is unavoidable thata torque (running resistance of the wheel supporting rolling bearingunit) required to turn the hub 7 (or 7 a) is increased because the sealrings 16 a, 16 b (or 16 c, 16 d) are provided to the opening portions onboth ends of the internal space in which the balls 14, 14 are provided.Meanwhile, for example, as set forth in JP-A-2001-241450, the structurein which, in order to isolate the internal spaces from the externalspace, the one end side of the internal space is closed by the cap andalso the seal ring is provided only to the other end side in the axialdirection is well known in the prior art.

However, in the case of the wheel supporting rolling bearing unit setforth in JP-A-2001-241450, because the running resistance of the sealring is not always low, it is impossible to sufficiently reduce therolling resistance of the wheel supporting rolling bearing unit. As aresult, since the running performances, mainly the accelerationperformance and the fuel consumption performance, of the vehicle intowhich the wheel supporting rolling bearing unit is incorporated becomeworsen, improvement in the running performances is desired in view ofthe recent trend toward the energy saving. The technology to reduce thesliding resistance between the seal member and the sliding portion ofthe counter member by mixing plastic fine grains which are impregnatedwith the lubricant into the rubber composition constituting the sealmember is known in JP-A-8-319379. However, no description is given inJP-A-8-319379 that suggests getting of the high-performance structure asa whole by applying the above rubber composition to the wheel supportingrolling bearing unit.

In addition, as the structure that reduced the running torque of therolling bearing unit by reducing the resistance of the seal-ringproviding portion, improvement in the inference of the seal lip as setforth in JP-A-10-252762 was considered in the prior art.

In the case of the wheel supporting rolling bearing unit as the subjectof the present invention, even though the running torque should bereduced, the structure capable of keeping the wheel supporting rigidityto ensure the controllability and also preventing sufficiently theentering of the foreign matter into the internal space of the rollingbearing unit to ensure the durability of the rolling bearing unit isneeded. In other words, the supporting rigidity must be assured byenhancing the rigidity of the rolling bearing unit to ensure thecontrollability, nevertheless the rolling resistance of respectiverolling members is increased if a preload applied to respective rollingmembers is increased simply to enhance the rigidity, whereby the runningtorque cannot be reduced. Also, in case it is considered only that thesliding resistance of the seal ring is lowered simply, prevention of theentering of the foreign matter into the internal space of the rollingbearing unit cannot be sufficiently attained and thus the durabilitycannot be sufficiently assured.

A wheel supporting rolling bearing unit of the present invention hasbeen made in view of such circumstances, and realizes a structure thathas a high rigidity, an excellent durability, and a low running torque.

DISCLOSURE OF THE INVENTION

A wheel supporting rolling bearing unit of the present inventioncomprises a stationary side raceway ring, a rotary side raceway ring, aplurality of balls, and a seal ring, as in the above wheel supportingrolling bearing unit known in the prior art.

The stationary side raceway ring is supported/fixed on a suspensionsystem in use.

The rotary side raceway ring supports/fixes a wheel in use.

A plurality of balls are provided between a stationary side racewaysurface and a rotary side raceway surface, each of which has acircular-arc sectional shape, on mutually opposing peripheral surfacesof the stationary side raceway ring and the rotary side raceway ring.

The seal ring seals only one opening portion out of opening portions onboth end portions of a space, in which the balls are provided, betweenthe mutually opposing peripheral surfaces of the stationary side racewayring and the rotary side raceway ring.

The other raceway ring, which is positioned inside in a radialdirection, out of the stationary side raceway ring and the rotary sideraceway ring consists of a main shaft member and an inner ring. Then,the main shaft member has a first inner ring raceway formed directly ina middle portion of an outer peripheral surface in an axial direction toserve as the stationary side raceway surface or the rotary side racewaysurface and a small-diameter stepped portion formed on one end portionof the outer peripheral surface in the axial direction. Also, the innerring on an outer peripheral surface of which a second inner ring racewayas the stationary side raceway surface or the rotary side racewaysurface is formed is fitted/fixed onto the small-diameter steppedportion.

The seal ring has two or three seal lips which are formed of elasticmaterial respectively and a top end edge of each of which slidinglycomes into contact with a counter surface.

In particular, in the wheel supporting rolling bearing unit of thepresent invention, an axial load to apply a preload to the balls is setto 1.96 to 4.9 kN.

Also, a rigidity factor is set to 0.09 or more.

Also, a torque required to relatively run the stationary side racewayring and the rotary side raceway ring at 200 min⁻¹ (200 RPM) based on afriction between the seal lip and a counter surface is set to 0.03 to0.2 N·m.

In addition, a torque required to relatively run the stationary sideraceway ring and the rotary side raceway ring at 200 min⁻¹ based on arolling resistance of the balls is set to 0.15 to 0.45 N·m.

The rigidity factor set forth in this specification means a ratio (R/Cr)of the rigidity R [kN·m/deg] of the wheel supporting rolling bearingunit to a radial dynamic rated load Cr [N] of the wheel supportingrolling bearing unit. The rigidity R in this case is represented by aninclination angle between both raceway rings when a moment load isloaded to the rotary side raceway ring in a situation that thestationary side raceway ring constituting the wheel supporting rollingbearing unit is fixed. For example, the rigidity is measured as shown inFIG. 13. In this case, FIG. 13 shows the condition in which the rigidityR of the wheel supporting rolling bearing unit 2 a shown in above FIG.12 is measured.

In the measuring operation, the outer ring 19 as the stationary sideraceway ring is secured to an upper surface of a fixing pedestal 26 anda base end portion (left end portion in FIG. 13) of a lever plate 27 iscoupled/fixed to the fitting flange 11 a of the hub 7 a as the rotaryside raceway ring. Then, the load is applied to the portion, which ispart from a center of rotation of the hub 7 a by a distance Lcorresponding to a radius of rotation of the tire, for example, on theupper surface of the lever plate 27 to apply the moment load of 1.5 kN·mto the hub 7 a via the lever plate 27. Since the hub 7 a is inclinedfrom the outer ring 19 based on this moment load, this inclination angleis measured as an inclination angle [deg] of a clamp face 29 of thefitting flange 11 a to an upper surface 28 of the fixing pedestal 26.Then, the rigidity R [kN·m/deg] is derived by dividing the moment load(1.5 kN·m) by this inclination angle. Then, the above rigidity factor isderived by dividing the rigidity R by the radial dynamic rated load Cr[N] of the wheel supporting rolling bearing unit 2 a.

In the case of the wheel supporting rolling bearing unit of the presentinvention constructed as above, the running torque can be reducedsufficiently while assuring the necessary rigidity and the durability.

First, since the axial load to apply a preload to the balls is regulatedin a range of 1.96 to 4.9 kN, the running torque can be reducedsufficiently while maintaining the rigidity and the durability. If theaxial load is below 1.96 kN, the preload is not enough and then therigidity of the wheel supporting rolling bearing unit becomesinsufficient. Thus, the controllability of the vehicle into which thewheel supporting rolling bearing unit is incorporated becomes worse.

In contrast, if the axial load is in excess of 4.9 kN, the preload isapplied excessively (a contact pressure of the rolling contact portionis increased excessively) and then the rolling resistance (runningtorque) of the wheel supporting rolling bearing unit is increasedexcessively. Then, a heating value at the rolling contact portionbecomes too large, and then a temperature rise in the wheel supportingrolling bearing unit becomes conspicuous. Thus, the grease that issealed in the bearing unit is ready to deteriorate immediately. As aresult, the durability of the wheel supporting rolling bearing unit islowered. Also, as the result of excessive increase of the contactpressure of the rolling contact portion, the rolling fatigue life of thestationary side raceway ring, the rotary side raceway ring and rollingcontact surfaces of the balls is shortened, and the durability of thewheel supporting rolling bearing unit is lowered in this respect.

In contrast, in the case of the present invention, since the axial loadto apply the preload to the balls is regulated in a range of 1.96 to 4.9kN, the running torque can be reduced sufficiently while keeping therigidity and the durability.

Also, since the rigidity factor is set to 0.09 or more, thecontrollability of the vehicle into which the wheel supporting rollingbearing unit is incorporated can be secured while maintaining therigidity of the wheel supporting rolling bearing unit. Converselyspeaking, the controllability becomes worse if the rigidity factor isbelow 0.09.

Here, because the higher rigidity factor is preferable from a viewpointof assuring the controllability, an upper limit is not particularlydefined. Any high value of the rigidity factor may be employed if otherrequirements are satisfied. Meanwhile, as the approach that is normallytaken to enhance the rigidity factor, a value of the preload should beincreased or a pitch diameter of the balls or a pitch between the ballsarranged in double rows in the axial direction should be increased.

In this case, as described above, there is a limit to an increase of thepreload. Also, there is a limit to increases of the pitch diameter andthe pitch in the axial direction from a viewpoint of reduction in sizeand weight. Therefore, when the wheel supporting rolling bearing unit ismade of normal steel material (the outer ring and the hub are made ofS53C and the inner ring and the balls are made of SUJ2), an upper limitof the rigidity factor becomes almost 0.18. In this case, when a part(e.g., balls) or all of constituent elements of the wheel supportingrolling bearing unit are made of ceramics, the rigidity factor can beincreased unless the preload and the pitch diameter and the pitch in theaxial direction are increased. Therefore, in this case, the rigidityfactor can also be increased to exceed 0.18.

Also, since the torque required to relatively run the stationary sideraceway ring and the rotary side raceway ring at 200 min⁻¹ based on thefriction between the seal lip and the counter surface is set to 0.03 to0.2 N·m, the running torque can be reduced sufficiently while assuringthe durability of the wheel supporting rolling bearing unit.

That is, as the result of experiments made by the inventors of thepresent invention, it was understood that, insofar as the number of theseal lips is set to two or three, a validity of the sealing performancecan be decided based on a magnitude of the running resistance of theseal ring irrespective of the structure of the seal ring. At the sametime, it was understood that, if the running resistance of the seal ringis set to 0.03 N·m or more, the required sealing performance can beattained.

In the case of the wheel supporting rolling bearing unit of the presentinvention, since the torque of 0.03 N·m or more is assured, the sealingperformance attained by the seal ring can be sufficiently assured bymaintaining sufficiently the contact pressure of the slide contactportion between top ends of the seal lips constituting the seal ring andthe counter surface. As a result, the durability of this wheelsupporting rolling bearing unit can be ensured by preventing effectivelythe entering of the foreign matter such as a muddy water, or the likeinto the inside of the wheel supporting rolling bearing unit. Converselyspeaking, if the contact pressure of the slide contact portion betweenthe top ends of the seal lips and the counter surface to reduce thetorque lower than 0.03 N·m, a function of preventing the entering of theforeign matter become insufficient and thus the durability of the wheelsupporting rolling bearing unit is lowered.

In contrast, if the torque exceeds 0.2 N·m, it is difficult to suppressthe running torque of the wheel supporting rolling bearing unit as awhole sufficiently low (0.65 N·m or less).

On the contrary, in the case of the present invention, since the torquerequired to relatively run the stationary side raceway ring and therotary side raceway ring at 200 min³¹ ¹ based on the friction betweenthe seal lip and the counter surface is regulated in a range of 0.03 to0.2 N·m, the running torque can be reduced while assuring thedurability.

In addition, since the torque required to relatively run the stationaryside raceway ring and the rotary side raceway ring at 200 min⁻¹ based onthe rolling resistance of the balls is regulated in a range of 0.15 to0.45 N·m, the running torque of the wheel supporting rolling bearingunit as a whole can be suppressed sufficiently low (0.65 N·m or less)while assuring the controllability and the durability.

If the torque is lessened below 0.15 N·m, the preload must be loweredconsiderably and, as described above, the rigidity of the wheelsupporting rolling bearing unit is not enough. Thus, the controllabilityof the vehicle into which the wheel supporting rolling bearing unit isincorporated is worsened.

In contrast, if the torque is enhanced so as to exceed 0.45 N·m, theincrease of the preload is brought about. Thus, as described above,reduction in the durability of the wheel supporting rolling bearing unitis caused owing to the deterioration of the grease and the reduction inthe rolling fatigue life, which are caused with an increase of theheating value at the rolling contact portion. Also, it is difficult tosuppress the running torque of the wheel supporting rolling bearing unitas a whole sufficiently low.

In contrast, in the case of the present invention, since the torquerequired to relatively run the stationary side raceway ring andthe-rotary side raceway ring at 200 min⁻¹ based on the rollingresistance of the balls is regulated in a range of 0.15 to 0.45 N·m, therunning torque can be reduced while assuring the controllability and thedurability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a first example of a structureserving as a subject of the present invention;

FIG. 2 is a sectional view showing a second example of the same;

FIG. 3 is a half-plane sectional view showing a third example of thesame;

FIG. 4 is a half-plane sectional view showing a fourth example of thesame;

FIG. 5 is a partial sectional view showing a first example of aparticular structure of a seal ring, which can be applied to the presentinvention;

FIG. 6 is a partial sectional view showing a second example of the same;

FIG. 7 is a partial sectional view showing a third example of the same;

FIG. 8 is a partial sectional view showing a fourth example of the same;

FIG. 9 is a partial sectional view showing a fifth example of the same;

FIG. 10 is a partial sectional view showing a first example of astructure that can reduce a slide resistance;

FIG. 11 is a sectional view showing a first example of a wheelsupporting rolling bearing unit, which is well known in the prior art,in a state that such unit is fitted into the suspension system;

FIG. 12 is a sectional view showing a second example of the same; and

FIG. 13 is a sectional view showing a condition in which a rigidity ofthe wheel supporting rolling bearing unit is measured.

BEST MODE FOR CARRYING OUT THE INVENTION

First, four examples of a structure of a wheel supporting rollingbearing unit as the subject of the present invention will be explainedhereinafter. At first, FIG. 1 shows a structure, as a first example, inwhich the running torque can be reduced readily by introducingimprovements into the above structure shown in FIG. 11 while assuringthe sealing performance and the controllability. For this purpose, inthe case of the present example, an inside inner ring raceway 13 a isformed directly on an outer peripheral surface of the middle portion ofa supporting shaft 4 a as a main shaft member. As a result, thestructure in which not only the rigidity can be assured not to renderthe manufacturing operation troublesome but also the foreign matters canbe prevented from entering through the fitting portion between theinside inner ring 5 and the supporting shaft 4, already considered inthe conventional structure shown in FIG. 11, can be obtained. Also, theoutside seal ring 16 a incorporated into the conventional structureshown in FIG. 11 is removed, and only the inside seal ring 16 b isprovided.

In the case where the present invention is applied to such structure, anaxial load to apply the preload to the balls 14, 14 is set to 1.96 to4.9 kN by regulating a torque that is applied to tighten the nut 6 beingscrewed onto the outer end portion of the supporting shaft 4 a. Then, atorque (rolling resistance) that is required to turn the hub 7 aroundthe supporting shaft 4 a at 200 min⁻¹ is set to 0.15 to 0.45 N·m. Thistorque is increased as the axial load is increased if otherspecifications are set similarly. Therefore, in order to set the torqueclose to a lower limit (0.15 N·m), the axial load is set close to alower limit (1.96 kN). When the torque is set to an upper limit (0.45N·m), a value that is close to an upper limit (4.9 kN) can be employedas the axial load. In this case, if other requirements such as arigidity factor described later, etc. can be satisfied, there is nonecessity that the value close to the upper limit must always beemployed as the axial load. For instance, if radii of curvature ofsectional shapes of the outer ring raceways 10 a , 10 b and the innerring raceways 13, 13 a are set close to 50% of the diameter of the balls14, 14, the rigidity factor can be secured and also the value close tothe lower limit or the middle value can be employed as the axial load.

Also, the axial load is set to 1.96 to 4.9 kN and the rigidity factor isset to 0.09 or more.

Further, the running resistance (torque) of the inside seal ring 16 b isregulated in a range of 0.03 to 0.2 N·m. The entering of the foreignmatter such as muddy water, or the like into the spaces in which theballs 14, 14 are provided is prevented by the seal ring 16 b and the cap17 deposited onto the outer-end opening portion of the hub 7 as asealing member. Structures of other portions are similar to the aboveconventional structure shown in FIG. 11. In this case, the cap 17 isfitted/fixed into the opening portion on the outer end of the hub 7 as atight fit. A clearance is formed between the cap 17 and a top endsurface (outer end surface) of the supporting shaft 4 a, and thus thecap 17 never slidingly comes into contact with the supporting shaft 4 awhen the hub 7 is turned.

Next, FIG. 2 shows a second example of the wheel supporting rollingbearing unit as the subject of the present invention. In the abovestructure shown in FIG. 1, the inner ring 5 is fixed by the nut 6 thatis screwed onto the outer end portion of the supporting shaft 4 a. Incontrast, in the present embodiment, the inner ring 5 is fixed to thesupporting shaft 4 a by pressing the outer end surface of the inner ring5 with the caulking portion 25 that is formed by elastically deformingan outer end surface of a supporting shaft 4 b as a main shaft memberoutward in the diameter direction. The axial load to apply the preloadis adjusted by a load applied when the caulking portion 25 is worked.Structures and operations of other portions are similar to the firstexample.

Next, FIG. 3 shows a third example of the wheel supporting rollingbearing unit as the subject of the present invention. In this example,the structure shown in above FIG. 12 is varied into a structure to whichthe present invention can be applied. For this purpose, in the case ofthe present example, an opening portion of the inner end of the outerring 19 is closed by a cap 17 a as a sealing member, and also the innerperipheral surface of the outer end portion of the outer ring 19 and theouter peripheral surface of the middle portion of the hub main body 22are isolated from each other by the seal ring 16 c. The cap 17 a isfitted/fixed into the opening portion on the inner end of the outer ring19 as a tight fit. A clearance is formed between the cap 17 a and thecaulking portion 25 that is formed on the inner end portion of the hub 7a as the main shaft member, and thus the cap 17 a in no ways slidinglycomes into contact with the hub 7 a when the hub 7 a is rotated. Theseal ring 16 d (FIG. 12) between the inner peripheral surface of theinner end portion of the outer ring 19 and the outer peripheral surfaceof the inner ring 23 is removed. Then, the running resistance of theseal ring 16 c is regulated into a range of 0.03 to 0.2 N·m. Theentering of the foreign matter such as muddy water, or the like into thespaces in which the balls 14, 14 are provided is prevented by the sealring 16 c and the cap 17 a. The axial load to apply the preload isadjusted by a load applied when the caulking portion 25 is worked.Structures and operations of other portions are similar to the firstexample. Structures of other portions are similar to the abovestructures in first and second examples and the above conventionalstructure shown in FIG. 12.

Next, FIG. 4 shows a fourth example of the wheel supporting rollingbearing unit as the subject of the present invention. In this example,an inner end surface of the inner ring 23 that is fitted onto thesmall-diameter stepped portion 24 of a hub main body 22 a is pressed bya nut 30 that is screwed onto an external thread portion 39 provided tothe inner end portion of the hub main body 22 a as the main shaftmember. A shape of a cap 17 b deposited onto the opening portion of theinner end of the outer ring 19 is expanded to meet this arrangement, sothat an interference between the external thread portion 39 and the nut30 is prevented. Thus, the cap 17 b by no means slidingly comes intocontact with the external thread portion 39 and the nut 30 when the hubmain body 22 a is rotated. The axial load to apply the preload isadjusted by a torque applied when the nut 30 is tightened. Structures ofother portions are similar to the second example.

Next, five examples of a particular structure of a seal ring, which canbe applied to the present invention, will be explained with reference toFIGS. 5 to 9 hereunder. These five examples shown in FIGS. 5 to 9 show astructure that can be utilized as the inside seal ring 16 b in the firstand second examples of the wheel supporting rolling bearing unit shownin FIGS. 1 and 2 respectively.

First, a first example shown in FIG. 5 is a combinational seal ring inwhich an outer diameter-side seal ring 31 that is fitted/fixed into theinner end portion of the hub 7 (FIGS. 1 and 2) and an innerdiameter-side seal ring 32 that is fitted/fixed onto the portion locatednear the inner ends of the supporting shafts 4 a (FIG. 1), 4 b (FIG. 2)are combined with each other. This seal ring has three seal lips intotal, i.e., two on the inner diameter side and one on the outerdiameter side. In the case of such structure, in the prior art, thetorque (running resistance) required for the relative rotation betweenthe outer diameter-side and inner diameter-side seal rings 31, 32 was0.22 N·m or more. In contrast, in case the structure of the presentexample is applied to the present invention, the torque (runningresistance) required for the relative rotation between the outerdiameter-side and inner diameter-side seal rings 31, 32 based on thefriction between top end edges of three seal lips and counter surfaces(surface of the core bar) is regulated in a range of 0.03 to 0.2 N·m bythe method described later, for example.

Next, a second example shown in FIG. 6 is a combinational seal ring inwhich a seal ring 33 that is fitted/fixed into the inner end portion ofthe hub 7 (FIGS. 1 and 2) and a slinger 34 that is fitted/fixed onto theportion located near the inner ends of the supporting shafts 4 a (FIG.1), 4 b (FIG. 2) are combined with each other. The seal ring 33 hasthree seal lips. In the case of the present example, the torque (runningresistance) required for the relative rotation between the seal ring 33and the slinger 34 based on the friction between top end edges of thesethree seal lips and the surface of the slinger 34 is regulated in arange of 0.03 to 0.2 N·m by the method described later, for example.

Next, a third example shown in FIG. 7 is a combinational seal ring inwhich a seal ring 35 a that engages with the inner peripheral surface ofthe inner end portion of the hub 7 (FIGS. 1 and 2) and a seal ring 35 bthat engages with the outer peripheral surface of the portion locatednear the inner ends of the supporting shafts 4 a (FIG. 1), 4 b (FIG. 2)are combined with each other. In the case of the present example, thisseal ring has three seal lips in total, i.e., two on the seal ring 35 aengaging with the hub 7 side and one on the seal ring 35 b engaging withthe supporting shafts 4 a, 4 b side. In the case of such presentexample, the torque (running resistance) required for the relativerotation between the hub 7 and the supporting shafts 4 a, 4 b based onthe friction between top end edges of these three seal lips and countersurfaces (inner peripheral surface of the hub 7, outer peripheralsurface of the supporting shafts 4 a, 4 b, surface of the core bar) isregulated in a range of 0.03 to 0.2 N·m.

Next, in a seal ring shown in FIG. 8, top end edges of two seal lipsprovided to a seal ring 36 that is fitted into the inner end portion ofthe hub 7 (FIGS. 1 and 2) come into contact with the outer peripheralsurface of the portion located near the inner ends of the supportingshafts 4 a (FIG. 1), 4 b (FIG. 2). In the case of such present example,the torque (running resistance) required for the relative rotationbetween the seal ring 36 and the supporting shaft 4 a based on thefriction between top end edges of two seal lips and the outer peripheralsurface of the inner end portions of the supporting shafts 4 a, 4 b isregulated in a range of 0.03 to 0.2 N·m.

Next, a seal ring 37 shown in FIG. 9 shows a structure that can beutilized as the inside seal ring in FIG. 2, and also can be utilized asa seal ring that is provided between the inner peripheral surface of theouter end portion of the outer ring 19 (FIGS. 3 and 4) and the outerperipheral surface of the middle portion of the hub main bodies 22 (FIG.3), 22 a (FIG. 4) in the wheel supporting rolling bearing unit shown inFIGS. 3 and 4 in the third and fourth examples. In this seal ring 37,three seal lips are provided to the core bar that can be fitted/fixedinto the outer end portion of the outer ring 19, and the top end edgesof these seal lips are brought into contact with the inside surface ofthe fitting flange 11 a (FIGS. 3 to 4) or the curved-surface portionthat connects continuously this inside surface and the outer peripheralsurfaces of the hub main bodies 22, 22 a. In the case of such presentexample, the torque (running resistance) required for the relativerotation between the seal ring 37 and the surfaces of the hub mainbodies 22, 22 a based on the friction between top end edges of threeseal lips and the surfaces of the hub main bodies 22, 22 a is regulatedin a range of 0.03 to 0.2 N·m.

In this event, as the method of reducing the torque required for therelative rotation between the seal ring and the counter member when anyone of the above seal rings is employed, there are methods {circlearound (1)} to {circle around (4)} described as follows, for example.These methods may be employed solely or in combination respectively.

{circle around (1)} To reduce a thickness of the seal lip.

In this case, the rigidity of the seal lip is lowered, then a contactpressure of the contact portion between the top end edge of the seal lipand the counter surface is lowered, and then the torque can be reduced.

{circle around (2)} To eliminate substantially an interference of thetop end edge of the seal lip, which is arranged closest to the internalspace in which the balls are provided, out of thee seal lips.

In this case, the concerned seal lip and the counter surface constitutea labyrinth seal. Then, a frictional resistance of the concerned seallip becomes 0.

{circle around (3)} To use the material having a small frictionalresistance rather than the nitrile rubber that is normally used atpresent, as the elastic material constituting the seal lip.

As the elastic material that has a small frictional resistance and isavailable in this case, as set forth in above Patent Literature 3(JP-A-8-319379), the material obtained by mixing the plastic finegrains, which are impregnated with the lubricant, into the rubbercomposition constituting the seal member may be employed.

{circle around (4)} To devise a sectional shape of the seal lip.

As the sectional shape that is available in this case, the sectionalshape in the prior invention disclosed in Patent Application No.2002-71338, for example, may be considered. In the structure in theprior invention, as shown in FIG. 10, a thickness of a seal lip 38 b ofthree seal lips 38 a, 38 b, 38 c is reduced gradually from the base endportion toward the middle portion and then is increased from the middleportion toward the top end portion. Also, a thickness of the seal lip 38b is maximized at a part of the portion located near the top end. Inaddition, the shape is decided to satisfy0.80d₁≦d₂≦0.98d₁, and 0.1S₂≦S₁≦0.5S₂where d₁ is a thickness of the base end portion of the seal lip 38 b, d₂is a thickness of a minimum thickness portion in which a thickness isminimized in the middle portion, S₁ is a sectional area of a portionextended from the base end portion to the minimum thickness portion inthe axial direction, and S₂ is an area of a portion extended from theminimum thickness portion to the top end edge.

EXAMPLES

Next, results of experiments to check advantages of the presentinvention will be explained hereunder. In first Experiment, therelationship between the running resistance (seal torque) of the sealring as a single body and the sealing performance was obtained in fivetypes of seal rings shown in FIGS. 5 to 9. Adjustment of the sealingtorque was executed by adjustment of the interference (an amount ofelastic deformation) of the seal lip, adjustment of the thickness of theseal lip, exchange of the elastic material, and adjustment of thecontact conditions to the counter surface. Then, six types of seal ringswhose seal torque is 0 to 0.15 N·m were manufactured with respect toabove five types of seal rings respectively. Then, respective seal ringswere incorporated into the wheel supporting rolling bearing unit shownin FIG. 1 or FIG. 3 to take a muddy-water entering test. In thismuddy-water entering test, as one cycle, the step of running relativelythe seal rings and the member having the counter surface while pouringthe muddy water on the area, in which the seal ring is provided, at arate of 3000 cc/min was continued for 17 hours and then the step ofdrying the concerned area by stopping the rotation and the pouring ofthe muddy water was continued for 3 hours. These steps were repeated in20 cycles every sample. Also, the lubrication of the wheel supportingrolling bearing unit was executed by injecting a grease whose viscosityis 10 to 14 cSt (10×10⁻⁶ to 14×10⁻⁶ m²/s). The hubs 7, 7 a were revolvedat 200 min⁻¹ in the environment of 20° C.

Results of the experiment executed under such conditions are given inTable 1.

TABLE 1 Running Resistance [N · m] FIG. 5 FIG. 6 FIG. 7 FIG. 8 FIG. 9 0X X X X X 0.01 Δ Δ X X Δ 0.03 ◯ ◯ ◯ ◯ ◯ 0.06 ◯ ◯ ◯ ◯ ◯ 0.10 ◯ ◯ ◯ ◯ ◯0.15 ◯ ◯ ◯ ◯ ◯

Here, in Table 1, a mark “X” indicates the fact that a large amount ofmuddy water entered into the internal space in which the grease issealed, a mark “Δ” indicates the fact that a small amount of muddy waterentered into the same, and a mark “◯” indicates the fact that theentering of the muddy water was not observed. It is appreciated based onsuch experimental results that, if the seal torque is in excess of 0.03N·m, the entering of the muddy water can be prevented by all seal rings.

Next, second to fifth Experiments that were executed by incorporatingthe seal ring 37 shown in FIG. 9 into the wheel supporting rollingbearing unit shown in FIG. 4 to know the influences of the seal torque(running resistance), the axial load to apply the preload, the rollingresistance, and the rigidity factor upon the controllability, therunning torque of the overall rolling bearing unit, and the durabilitywill be explained with reference to Tables 2 to 5. In Tables 2 to 5, amark “X” indicates the fact that the problem arose in practical use inany respect, a mark “Δ” indicates the fact that the problem slightlyarose in practical use in any respect, and a mark “◯” indicates the factthat no problem arose in all respects. Here, second to fifth Experimentswere carried out three times under the same conditions respectively.

First, Table 2 gives results of second Experiment that was executed toknow the influence of the seal torque upon the running torque of theoverall rolling bearing unit and the durability. This Experiment wasexecuted at a rotational speed of 200 min⁻¹.

TABLE 2 Seal torque[N · m] Evaluation 0 X X X 0.01 Δ X Δ 0.03 ◯ ◯ ◯ 0.06◯ ◯ ◯ 0.10 ◯ ◯ ◯ 0.15 ◯ ◯ ◯ 0.20 ◯ ◯ ◯ 0.33 X X X 0.45 X X X

As the result of second Experiment given in Table 2, it was understoodthat, if the seal torque is in a range of 0.03 to 0.20 N·m, thesatisfactory performance can be obtained in both respects of the runningtorque of the overall rolling bearing unit and the durability. Incontrast, if the seal torque is at 0 N·m and 0.01 N·m (small), theentering of the foreign matter such as the muddy water, or the like intothe internal space in which the balls 14, 14 are provided could notsufficiently prevented, and thus the problem arose in the respect ofassuring the durability. In contrast, if the seal torque is at 0.33 N·mand 0.45 N·m (large), the running torque of the overall rolling bearingunit could not be suppressed low.

Next, Table 3 gives results of third Experiment that was executed toknow the influence of the axial load (preload) upon the rigidity of therolling bearing unit and the durability.

TABLE 3 Preload [kN] Evaluation 0.49 X X X 0.98 Δ Δ X 1.96 ◯ ◯ ◯ 2.94 ◯◯ ◯ 3.92 ◯ ◯ ◯ 4.90 ◯ ◯ ◯ 5.88 Δ Δ Δ 6.86 X X Δ

As the result of third Experiment given in Table 3, it was understoodthat, if the axial load is in a range of 1.96 to 4.90 N·m, thesatisfactory performance can be obtained in both respects of thecontrollability and the durability of the rolling bearing unit. Incontrast, if the axial load is at 0.49 kN and 0.98 kN (too small), therigidity of the rolling bearing unit was low and thus the sufficientcontrollability could not be assured. In contrast, if the axial load isat 5.88 kN and 6.86 kN (too large), the rolling resistance was increasedand the durability of the rolling bearing unit was degraded.

Next, Table 4 gives results of fourth Experiment that was executed toknow the influence of the rolling resistance upon the rigidity of therolling bearing unit and the durability. This Experiment was executed ata rotational speed of 200 min⁻¹.

TABLE 4 Rolling Resistance [N · m] Evaluation 0.10 X X X 0.12 X Δ Δ 0.15◯ ◯ ◯ 0.25 ◯ ◯ ◯ 0.35 ◯ ◯ ◯ 0.45 ◯ ◯ ◯ 0.55 Δ X Δ 0.65 X X X

As the result of fourth Experiment given in Table 4, it was understoodthat, if the rolling resistance is in a range of 0.15 to 0.45 N·m, thesatisfactory performance can be obtained in both respects of thecontrollability and the durability of the rolling bearing unit. Incontrast, if the rolling resistance is at 0.10 N m and 0.12 N·m (toosmall), the rigidity of the rolling bearing unit was low and thus thesufficient controllability could not be assured. In contrast, if therolling resistance is at 0.55 N·m and 0.65 N·m (too large), thedurability of the rolling bearing unit was degraded.

In addition, Table 5 gives results of fifth Experiment that was executedto know the influence of the rigidity factor upon the rigidity of therolling bearing unit.

TABLE 5 Rigidity factor Evaluation 0.07 X X X 0.08 X Δ X 0.09 ◯ ◯ ◯ 0.10◯ ◯ ◯ 0.15 ◯ ◯ ◯ 0.18 ◯ ◯ ◯

As the result of fifth Experiment given in Table 5, it was understoodthat, if the rigidity factor is 0.09 or more, the satisfactoryperformance of the controllability can be obtained. In contrast, if therigidity factor is at 0.07 and 0.08, the rigidity of the rolling bearingunit was low and thus the sufficient controllability could not beassured. In this case, as described above, the higher rigidity factor isbetter insofar as other requirements are satisfied.

Next, Table 6 gives results of Experiment that was executed to know theinfluences of the seal torque and the rolling resistance upon therunning torque of the rolling bearing unit as a whole. This Experimentwas executed at a rotational speed of 200 min⁻¹.

TABLE 6 Rolling Resistance Seal torque [N · m] [N · m] 0.15 0.2 0.330.35 ◯ ◯ Δ 0.45 ◯ ◯ X 0.55 Δ X X

In Table 6, a mark “X” indicates the fact that the running torque waslarge as a whole, a mark “Δ” indicates the fact that the running torquewas slightly large, and a mark “◯” indicates the fact that the runningtorque was small. As apparent from this Table 6, the present inventionin which both the seal torque and the rolling resistance are suppressedto 0.2 N·m or less and 0.45 N·m or less respectively could suppress as awhole the running torque low such as 0.65 N·m or less.

Four examples of the specifications of the wheel supporting rollingbearing unit belonging to the technical scope of the present inventionare given in Table 7.

TABLE 7 Rolling torque Ball Inter-row Contact Preload [N · m] (measuredRigidity Diameter PCD distance angle No [kN] at 200 min⁻¹) factor [mm][mm] [mm] [deg] 1 1.96 0152 0.093 φ 9.525 46 24 40 4.9 0.448 0.102 φ9.525 46 24 40 2 1.96 0.150 0.091 φ 12.7 51 35 40 4.9 0.445 0.100 φ 12.751 35 40

In Table 7, the ball diameter denotes a diameter of the ball, the PCDdenotes a pitch diameter of a ball sequence of these balls, theinter-row distance denotes a pitch (center distance between the balls)of the ball sequences arranges in double rows in the axial direction,and the contact angle denotes a contact angle between the balls and theinner ring raceway and the outer ring raceway.

Also, the influence of the contact angle upon the rigidity factor isgiven in Table 8. It was appreciated from Table 8 that the rigidityfactor becomes small as the contact angle becomes small.

TABLE 8 Contact Ball Inter-row Preload angle Rigidity Diameter PCDDistance [kN] [deg] factor [mm] [mm] [mm] 1.96 40 0.093 φ 9.525 45 241.96 35 0.089 φ 9.525 45 24

The present invention is explained in detail with reference toparticular embodiments, but it is apparent for the person skilled in theart that various variations and modifications can be applied withoutdeparting from a spirit and a scope of the present invention.

The present application was filed based on Japanese Patent Application(Patent Application No. 2002-261195) filed on Sep. 6, 2002 and thecontents thereof are incorporated herein by the reference.

INDUSTRIAL APPLICABILITY

Since the wheel supporting rolling bearing unit of the present inventionis constructed and acts as described above, such bearing unit cancontribute to the improvement in the running performances of thevehicle, mainly the controllability, the acceleration performance, andthe fuel consumption performance, by reducing the running torque of thehub, which rotates together with the wheel, while assuring the rigidityand the durability.

An example of a trial calculation to improve the fuel consumptionperformance will be explained hereunder. The running resistance of thewheel supporting rolling bearing unit having the above structure shownin FIGS. 1 to 4 was almost 1 N·m in the prior art. In contrast, therunning resistance of the wheel supporting rolling bearing unit of thepresent invention is in a range of 0.18 to 0.65 N·m. That is, therunning resistance is lowered by 35% or more rather than the prior art.It is considered that, if the running resistance of the wheel supportingrolling bearing unit is lowered by 10%, the fuel consumption (fuelconsumption ratio) can be improved by about 0.1%. Therefore, supposethat the car the fuel consumption of which is about 10 km/L travels100,000 km a year, the fuel can be saved by about 35 to 82 L a year byemploying the wheel supporting rolling bearing unit of the presentinvention. Suppose that 1,000,000 cars travel in this country, the fuelthat can be saved a year reaches 35,000,000 L to 82,000,000 L. Inaddition, it is possible to say that the industrial usability isextremely high because the fuel consumption can be improved not to causeother disadvantages.

1. A wheel supporting rolling bearing unit comprising: a stationary sideraceway ring supported and fixed on a suspension system in use; a rotaryside raceway ring for supporting and fixing a wheel in use; a pluralityof balls provided between a stationary side raceway surface and a rotaryside raceway surface, each of which has a circular-arc sectional shape,on mutually opposing peripheral surfaces of the stationary side racewayring and the rotary side raceway ring; and a seal ring for sealing onlyone opening portion out of opening portions on both end portions of aspace in which the balls are provided between the mutually opposingperipheral surfaces of the stationary side raceway ring and the rotaryside raceway ring; wherein the other raceway ring, which is positionedinside in a radial direction, out of the stationary side raceway ringand the rotary side raceway ring consists of a main shaft member and aninner ring, the main shaft member has a first inner ring raceway formeddirectly in a middle portion of an outer peripheral surface in an axialdirection to serve as the stationary side raceway surface or the rotaryside raceway surface and a small-diameter stepped portion formed on oneend portion of the outer peripheral surface in the axial direction, andthe inner ring on an outer peripheral surface of which a second innerring raceway as the stationary side raceway surface or the rotary sideraceway surface is formed is fitted/fixed onto the small-diameterstepped portion, the seal ring has two or three seal lips which areformed of elastic material respectively and a top end edge of each ofwhich slidingly comes into contact with a counter surface, wherein anaxial load to apply a preload to the balls is set to 1.96 to 4.9 kN, arigidity factor is set to 0.09 or more, a torque required to relativelyrun the stationary side raceway ring and the rotary side raceway ring at200 min⁻¹ based on a friction between the seal lip and a counter surfaceis set to 0.03 to 0.2 N·m, and a torque required to relatively run thestationary side raceway ring and the rotary side raceway ring at 200min⁻¹ based on a rolling resistance of each ball is set to 0.15 to 0.45N·m.
 2. A wheel supporting rolling bearing unit according to claim 1,wherein the inner ring is pressed by a caulking portion, which is formedby elastically deforming one end portion of the main shaft memberoutward in the radial direction, at one end surface.